t0021: avoid getting filter killed with SIGPIPE

The fake filter did not read from the standard input at all,
which caused the calling side to die with SIGPIPE, depending
on the timing.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t0021-conversion.sh

@@ -100,6 +100,7 @@ test_expect_success expanded_in_repo '
 test_expect_success 'filter shell-escaped filenames' '
 	cat >argc.sh <<-EOF &&
 	#!$SHELL_PATH
+	cat >/dev/null
 	echo argc: \$# "\$@"
 	EOF
 	normal=name-no-magic &&